The combination of metal ignition aids with a discharge vessel consisting of quartz glass or of sodium-permeable ceramic has until now only been possible with significant restrictions since the metal parts guided past the discharge vessel cause the sodium to diffuse out of the discharge vessel. In order to avoid this emergence of sodium, caused by metal ignition aids, some sometimes complex countermeasures have been proposed. For example, the galvanic contact can be isolated after starting by bimetallic-element switches, for example U.S. Pat. No. 5,757,137, or external switches, for example EP-A 1 162 865, in order to prevent the emergence of sodium. It is also known from U.S. Pat. No. 5,001,360 to plug a ceramic tube over the power supply line running parallel to the burner in order to prevent photoionization from the feed line. The problem with this consists in that the entire feed line is not shielded via the ceramic tube and the remaining free parts of the feed line can nevertheless cause the emergence of sodium as a result of photoionization.